Cervical collars are applied if the mechanism of injury warrants it. If someone falls and strikes their head hard enough to knock them unconscious, then there is a risk that they have injured their neck. They should be placed in a c-collar and on a backboard as a precaution against spinal injury.
Some systems train their medics in a spinal rule out protocol. They may withhold spinal precautions if there doesn't appear to be an injury, but any altered level of consciousness after a possible neck injury calls for a collar and backboard.